1. Field of the Invention
The present invention relates to a process for preparing coatings from aqueous dispersions which rapidly dry physically and at the same time result in coatings that are resistant to the impact of stones and are stable to weathering, especially for the series lacquering of motor vehicles.
2. Description of the Prior Art
In the series coating of motor vehicles, multilayer coatings are generally used. Aqueous coating compositions are by increasing used to prepare these coatings. In the case of such modern coatings, demands are also made regarding application costs. The aim is to apply the coating using the smallest possible number of individual steps and such that the individual steps are as inexpensive as possible.
In the case of the usual multilayer coatings applied to motor vehicles, a stone impact protective coating and a filler coating, or a combination of the two coatings ("stone impact protective filler"), is applied to a metal surface which has initially been primer coated by cathodic electro-dipcoating (CDC). There are then applied to those coatings either a pigmented base coat and a clear top coat or, alternatively, a pigmented top coat.
The stone impact protective and/or filler coating levels out any uneven areas in the surface and, as a result of a high degree of elasticity and deformability, provides good resistance to the impact of stones. Previously, soft polyester- or polyether-polyurethanes and polyisocyanate or melamine crosslinking agents have been used for this coating. Before the base coat and clear top coat or finishing lacquer are applied, the stone impact protective filler is stoved. This is necessary in order to improve the condition of the finishing lacquer and to polish off the filler coating before the upper lacquer coatings are applied. After the clear lacquer or finishing lacquer is applied, stoving is then carried out again. A disadvantage of this process is that two expensive stoving operations are necessary. Lacquers that rapidly dry physically, i.e., without a stoving operation (such as coatings formulated from polyacrylates) do not have the required resistance to the impact of stones because they crosslink to form brittle films during the stoving operation.
An object of the present invention is to provide binders which are storage stable, have good resistance to the impact of stones and rapidly dry physically. It is an additional object of the present invention to provide binders which are light-fast and stable to weathering, especially in the case of finishing or base lacquers with moderate coverage or in areas in which no pigmented finishing or base lacquer is applied at all. It is a further object of the present invention to provide an aqueous coating composition that combines the fundamental properties of a stone impact protective filler and a finishing lacquer binder and, in addition, that rapidly dries physically.
Surprisingly, it has been found that this object can be achieved by the use of a combination of specific aqueous polyols and blocked polyisocyanates. The polyol dispersions according to the invention are characterized in that they contain graft copolymers based on polyesters-polyacrylates in which the polyester component is based on long-chain aliphatic monocarboxylic acids and .alpha.,.beta.-unsaturated carboxylic acids. The suitability of the polyol dispersions according to the invention for automotive fillers is surprising because previously when stoving lacquers containing a high proportion of polyacrylate were used for automotive fillers, they did not satisfy the high property requirements with respect to elasticity and resistance to the impact of stones.
Coatings and/or filler coatings based on aqueous binders that are resistant to the impact of stones are described, for example, in EP-A 330,139. It is claimed that a polyacrylate can be mixed with an OH- and COOH-functional polyester. No examples are directed to this embodiment and graft copolymerization of the polyacrylate onto a polyester as grafting base is not described. The claimed dispersions of acid-functional polyesters are known to have poor storage stability, since they undergo rapid chemical degradation as a result of cleavage of the ester bonds as described, e.g., in Jones, T. E.; McCarthy, J. M., J. Coatings Technol. 76 (844), p. 57 (1995).